1. Field of the Invention
This invention relates generally to a method and apparatus for rapidly charging a battery by monitoring a parameter associated with the battery, such as voltage, and, in particular, by varying the effective sample rate of the monitored parameter during different phases of the charging process in accordance with the relative rate of change in the value of the monitored parameter.
2. Discussion of the Related Art
For a variety of well known reasons, such as reducing the number of disposable batteries and reducing the possibility of being without batteries having adequate charge, it is desirable to provide a rechargeable battery or battery pack. It is further desirable to be able to recharge a chargeable battery or battery pack as fast as possible. Battery chargers which recharge chargeable batteries very rapidly, typically on the order of 15 minutes, are known in the art. Because of the high amount of current required to be applied to a rechargeable battery in an uncharged state in order to recharge it rapidly, it is extremely important to terminate the charging procedure before the battery pack is overcharged. Charging beyond the full battery charge leads to battery deterioration and, over an extended number of overchargings, to battery failure.
A number of different charging systems are known in the art to rapidly charge rechargeable batteries which include methods for detecting full battery charge in order to eliminate, or substantially decrease, the possibility of overcharging the batteries. Examples of the more effective prior art rapid charging systems are disclosed in U.S. Pat. Nos. 4,388,582 and 4,392,101, both issued to Saar et al., herein incorporated by reference and assigned to the same assignee as the present invention. The specific subject matter of the Saar et al. patents is directed to a quick charging technique which analyzes the charging of a battery by noting inflection points which occur in a charging curve representing the voltage increase of the battery with respect to time. The inflection points indicate areas in the charging curve which reliably separate different regions of electrochemical changes in the battery during the charging sequence. By determining the specific inflection points in the charging curve, it is possible to accurately terminate the rapid charging when the battery receives full charge.
Although the inflection point analysis technique has met with overall success, there is still room for improvement of these types of devices. In one specific area, random noise in the battery voltage signal monitored during the charging sequence may prematurely indicate an inflection point in the charging sequence, and possibly even prematurely indicate maximum charge. More particularly, as the charging system measures the battery voltage during the charging sequence in order to determine the slope of the charge curve to detect the inflection points, random noise may indicate a change in the slope which appears to but does not represent an actual inflection point. In regions of minimal voltage charge in the charging sequence, noise will obviously have the most destructive effects. Consequently, it is generally desirable to provide a relatively long sampling rate in regions where the voltage curve is relatively flat in order to increase the effective signal-to-noise ratio of the sampled signal.
Once the charging sequence enters a high slope region, the voltage increases fairly rapidly such that a relatively long sampling rate is no longer desirable and may in fact cause the optimum termination point to be missed. Consequently, it is desirable to have a relatively fast sample rate in the high slope region. Since the prior art charging curve analysis techniques have generally maintained a constant sampling rate throughout the charging process, there has been a tradeoff between an appropriate sample rate for the flat slope region and an appropriate sample rate for the high slope region.
Accordingly, it is the primary object of the present invention to overcome these disadvantages and provide a fast charging device which incorporates a relatively long effective sample rate in the flat slope region of a charging sequence and a relatively short effective sample rate in the high slope region.